High-side sensing of zero inductor current for step down DC-DC converter

ABSTRACT

A DC to DC converter circuit includes circuitry for generating a PWM waveform signal at a phase node of a DC to DC converter responsive to an input voltage and a monitor output voltage. The circuitry further includes a high side switching transistor connected between the input voltage and a phase node and a low side switching transistor connected between the phase node and ground. An output filter is connected to the circuitry for generating the PWM waveform signal. The output filter includes an inductor having a first side connected to the phase node and a second side connected to an output voltage node. Detection circuitry detects zero current crossings in the inductor responsive to a voltage across the high side switching transistor and a voltage across the low side switching transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims benefit of U.S. Application Ser. No. 61/033,259,filed Mar. 3, 2008, and entitled HIGH-SIDE SENSING OF ZERO INDUCTORCURRENT FOR STEP DOWN DC-DC CONVERTER, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to DC to DC converters, and moreparticularly, to the sensing of zero inductor currents using bothhigh-side and low-side switching transistors within a DC to DCconverter.

BACKGROUND

DC to DC voltage regulators are used for maintaining a consistent outputvoltage for application to digital and analog circuitries within variouscircuit devices. An applied input voltage may vary for various reasons.However, despite these variances in the input voltage it is necessary tomaintain the voltage applied to various internal circuitries at aconsistent level that does not adversely effect the operation of theassociated circuit. A DC to DC voltage converter adequately providesthis functionality. The DC to DC voltage converter includes an inductortherein having a current passing there through. Effectively sensing thezero crossing of this inductor current within the DC to DC converter isan important feature for synchronous DC to DC converters.

The zero crossing detection of the inductor current is a critical factorthat the DC to DC converter needs in order to determine on/off state ofthe low side switching transistor in order to save energy and improvethe overall efficiency at the DC to DC converter. Improving theefficiency will enable the extension of the battery life of theassociated electrical circuit. In prior art environments, the inductorcurrent zero crossing is detected by sensing voltage through the lowside switching transistor. Typically, when determining zero inductorcurrent, a determination is made when the current decreases from aparticular value to zero. The inductor current decreases only when thelow side switching transistor is turned on. This makes traditional zerocrossing sensing techniques applicable only during the on condition ofthe low side switching transistor.

However, the use of the low side switching transistor to determine aconductor current zero crossing has certain limitations. One limitationarises when the switching frequency of the switching transistors of theDC to DC converter increases. As the switching frequency increases, theswitching cycle becomes shorter. Therefore for the same duty cycle, boththe turn on and the turn off time become smaller. At particular points,the turn on time of the low side switching transistor is so short thatthe length of time that the transistor is turned on is not sufficient toenable a sensing device to respond to the “on” state of the low sideswitching transistor. This causes a problem with using the low sideswitching transistor “on” condition as an indication of zero crossinginductor current sensing. Thus, there is a need for a improved methodfor detecting inductor current zero crossings in a DC to DC converter.

SUMMARY

The present invention, as disclosed and described herein, comprises inone aspect thereof a DC to DC converter including circuitry forgenerating a PWM waveform at a phase node of a DC to DC converterresponsive to an input voltage and a monitored output voltage. Thecircuitry further comprises a high side switching transistor connectedbetween the input voltage and the phase node and a low side switchingtransistor connected between the phase node and ground. An output filterconnected to the circuitry for generating the PWM waveform signal at thephase node includes an inductor having a first side connected to thephase node and a second side connected to the output voltage node.Detection circuitry detects zero current crossing within the inductorresponsive to a voltage across both the high side switching transistorand a voltage across the low side switching transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding, reference is now made to thefollowing description taken in conjunction with the accompanyingDrawings in which:

FIG. 1 is a functional block diagram of a voltage regulator circuit;

FIG. 2 is a schematic diagram of a voltage regulator with a PWMconverter circuit;

FIG. 3 is a schematic diagram of the circuitry for detecting a zeroinductor current within a step down DC to DC converter; and

FIG. 4 illustrates the wave forms associated with the operation of thecircuit of FIG. 3.

DETAILED DESCRIPTION

Referring now to the drawings, wherein like reference numbers are usedherein to designate like elements throughout, the various views andembodiments of high-side sensing of zero inductor current for step downdc-dc converter are illustrated and described, and other possibleembodiments are described. The figures are not necessarily drawn toscale, and in some instances the drawings have been exaggerated and/orsimplified in places for illustrative purposes only. One of ordinaryskill in the art will appreciate the many possible applications andvariations based on the following examples of possible embodiments.Although the preferred embodiment has been described in detail, itshould be understood that various changes, substitutions and alterationscan be made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

Referring now to the drawings, and more particularly to FIG. 1, there isillustrated a voltage regulator circuit 102. Every electronic circuit isdesigned to operate off of some type of voltage supply which is assumedto be constant. A voltage regulator provides this constant DC outputvoltage and contains circuitry that continuously holds the outputvoltage at the designed value regardless of changes in load current orinput voltage. A voltage regulator operates by using a voltagecontrolled current/voltage source 104 to force a fixed voltage to appearat the regulator output terminal 106. A sense/control circuitry 110monitors the output voltage, and adjusts the current source 104 to holdthe output voltage at the desired level. The design limit of the currentsource 104 defines the maximum load current the voltage regulator 102can source and still maintain regulation of the output voltage.

The output voltage is controlled using a feedback loop which requiressome type of compensation to assure loop stability. Most voltageregulators have built-in compensation and are completely stable withoutexternal components. Some regulators require some external capacitanceconnected from the output lead to ground to ensure regulator stability.Another characteristic of a voltage regulator is that it requires afinite amount of time to correct the output voltage after a change in aload current demand. The time lag defines the characteristic transientresponse of the voltage regulator, which is a measure of how fast thevoltage regulator returns to steady state conditions after a loadchange. Voltage regulation may be used in any number of electronicdevices to control an output voltage.

Referring now to FIG. 2, there is illustrated a voltage regulator withina PWM DC-DC converter circuit 202. The output voltage VOUT is regulatedto the reference voltage signal VREF applied to a positive input oferror amplifier 204. The error amplifier 204 output is compared with theoutput of oscillator 206, which generates a triangular waveform, at thePWM comparator 208. The output of the PWM comparator 208 is applied todriver circuits 210 and 212, which drive the gates of high sideswitching transistor 214 and low side switching transistor 216. Thisprocess provides a pulse width modulated waveform with an amplitude ofVIN at a phase node 218 connected to a first side of inductor 220. ThePWM waveform provided from phase node 218 is smoothed by an outputfilter consisting of inductor 220 and capacitor 222. The error amplifier204 has an input impedance ZIN 224 and an FB (Feed back) pin impedanceZFB at 226. The input impedance 224 and the FB pin impedance 226comprise a compensation loop for the error amplifier 204.

Referring now to FIG. 3, there is illustrated a schematic diagram of thecircuitry for detecting the inductor current zero crossings using eachof the high side switching transistor 214 and the low side switchingtransistor 216 described previously with respect to FIG. 2. The highside switching transistor 214 is connected between the input voltagePVIN and the phase node 218. As illustrated previously in FIG. 2, thephase node 218 is the node connected to the inductor 220 through whichthe inductor current zero crossing is to be detected. The low sideswitching transistor 216 is connected between the phase node 218 andground. A first comparator 308 has its inputs connected to the phasenode 218 and ground respectively. This enables detecting of the voltageacross the low side switching transistor to determine its on/off state.The output of the comparator 308 is connected to a first input of an ORgate 310. A second comparator 312 has its inputs connected to the inputvoltage node PVIN and to the phase node 218. This enables detecting ofthe voltage across the high side switching transistor to determine itson/off state. The output of the OR gate 310 is connected to a counter311. The counter counts the number of clock cycles that the outputsignal goes high. If the counter 311 monitors eights continuous clockcycles high for the output signal, this indicates that either the COMP 1or COMP 2 has detected zero crossings for eight continuous clock cycleswhich will initiate the pulse frequency modulation (PFM control mode).Thus, the output signal is used in the PWM mode (CCM mode) to initiatethe PFM control mode. The output of comparator 312 is connected to asecond input of the OR gate 310.

The higher the duty cycles or switching frequency of the DC to DCconverter, the narrower the pulse width provided across the low sideswitching transistor 216. The comparator 308 is connected to monitor thevoltage across the low side switching transistor 216 between the phasenode 218 and ground. When the inductor current reaches zero or is belowzero while the low side switching transistor is turned “on”, the voltageat the phase node 218 is equal to or higher than the ground voltage PGNDat the ground node. When this condition occurs, the output of thecomparator 308 goes to a logic high (“one”) level due to the value ofthe voltage at the phase node VPHS minus the voltage PGND at the groundnode changing from a negative value to a positive value. In this manner,an inductor current zero crossing is detected.

However, every comparator such as comparator 308 has speed limitationsand requires a certain amount of time in order to respond at the outputto changes within the input difference applied to the inputs of thecomparator 308. As the switching frequency or duty cycle of the DC to DCconverter increases (i.e. VOUT is close to VIN), the “on” time of thelow side switching transistor 216 becomes shorter and shorter. Thehigher the duty cycle/switching frequency of the DC to DC converter, thenarrower the pulse width applied to the switching transistors. At acertain point, the comparator 308 will have no time to respond tovoltage changes across the low side switching transistor 216 because ofthe short “on” time of the transistor 216. When the low side switchingtransistor 216 “on” time is shorter than the response time of thecomparator 308, there will be no detection of the voltage change acrossthe transistor 216 by the comparator 308. However, this shortcoming maybe overcome by monitoring the voltage across the high side switchingtransistor 214 using a second comparator 312.

When the response time of the comparator 308 is insufficient to detectthe “on” time of the low side switching transistor 216, the zero currentcrossing within the inductor 220 can not be detected and the DC to DCconverter can not change from PWM (pulse width modulation) mode to PFM(pulse frequency modulation) mode, even when the load current reacheszero. The addition of comparator 312 and OR gate 310 enables thisproblem to be overcome. The comparator 312 enables the inductor currentzero crossing point to also be detected during the “on” time of the highside switching transistor 214. When the inductor current is zero ornegative, the phase voltage VPHS at node 218 is equal to or higher thanthe input voltage PVIN, and the output of the comparator 312 istriggered to a logical high level (“one”) due to the change of the phasenode voltage VPHS minus the input node voltage VPVIN from negative topositive. This enables the inductor current zero crossing within theinductor 220 to be detected during the high side switching transistor214 “on” time.

The outputs of comparator 312 and comparator 308 are ORed together bythe OR gate 310 to generate an inductor current zero crossing leveldetector that measures zero crossing values across a broader frequencyand duty cycle range than merely the comparator 308 monitoring the lowside transistor 216. Thus, an indication of the zero current crossingwithin the inductor indicated by the voltage across either of the highside switching transistor 214 or the low side switching transistor 216may cause the output of the OR gate 310 to be triggered indicating thezero current crossing detection. When the duty cycle of the DC to DCconverter is high and comparator 308 does not have a sufficient responsetime to respond to voltage changes across the transistor 216, the longer“on” time of the high side switching transistor 214 due to its fixedswitching frequency will enable sufficient time for the comparator 312to detect voltage changes and respond to inductor current zerocrossings. The same conditions occur when the duty cycle is at a verylow level and comparator 312 has more time to detect inductor currentzero crossings. As described previously, when the output of the OR gate310 is determined to detect zero crossing for eight continuous clockcycles by the counter 311, this is used to initiate the PFM control modewithin the switching control circuitry of the PWM controller associatedwith the regulator. Comparator 312 and comparator 308 are never on atthe same time. Comparator 312 is on after the high-side switchingtransistor is turned on, while comparator 308 is turned on after thelow-side switching transistor is turned on. While the comparator 308 isdisabled, the output of the comparator is forced to a logical low level.When the comparator 312 is disabled, its output is also forced to alogical low level. The output of the OR gate 310 is connected to acounter 311. The counter counts the number of clock cycles that theoutput signal goes high. If the counter 311 monitors eight continuousclock cycles high for the output signal, this indicates, that either theCOMP 1 or COMP 2 has detected zero crossings for eight continuous clockcycles which will initiate the pulse frequency modulation (PFM controlmode). Thus the output signal is used in the PWM mode (CCM mode) toinitiate the PFM control mode. Thus, a complementary detection system isprovided.

Referring now to FIG. 4, there are illustrated the wave forms of thevarious signals at particular nodes within the circuit described withrespect to FIG. 3. The waveform I_(L) 402 represents the inductorcurrent through the inductor 220 of a DC to DC converter. The currentthrough the conductor is represented by a sawtooth type waveform thatgoes above and below a zero amp inductor current indicated generally bythe dashed line 404. The inductor current is increasing from times T₁one to times T₂ and is decreasing from time T₂ to time T₃. This patternrepeats throughout the waveform 402. A zero crossing of the inductorcurrent is indicating generally at 406. Of course numerous otheroccurrences of zero crossings within the waveform are also present.

The phase node voltage waveform VPHS is illustrated generally bywaveform 408. Additionally illustrated with respect to this waveform arethe input voltage which is indicated as a consistent voltage VPVIN 410and the ground voltage indicated by PGND 412. The voltage at the phasenode 218 is decreasing from times T₁ to times T₂. At time T₂ the voltagedrops below PGND 412. The voltage at the phase node 420 then beginsincreasing from time T₂ to time T₃. This process is then repeated.

The output voltage of comparator 308 is represented by voltage wave form414. The output of comparator 308 goes high at each instance of theinductor current crossing zero at times T₁, T₃, etc. The output ofcomparator 312 is indicated generally by the waveform 416. This pulsemonitors the voltage across the high side transistor and provides anindication when the phase voltage VPHS is equal to or higher than theinput voltage VPVIN. The OR gate output represented generally bywaveform 420, provides a wider pulse when both the outputs of thecomparator 308 and 312 have gone high responsive to a zero currentcrossing or additionally registers a narrower pulse responsive to theoutput of the comparator 312 when only the comparator 312 may detect thezero current crossing due to the frequency/duty cycle of the DC to DCvoltage inverter being to high. Therefore, using the above describedsystem and method, there are no duty cycle limitations to the circuitryas compared with prior art implementations. The circuitry can achieve atruly 100% duty cycle PWM to PFM transition.

It will be appreciated by those skilled in the art having the benefit ofthis disclosure that this high-side sensing of zero inductor current forstep down dc-dc converter provides an improved inductor current zerocrossing detector. It should be understood that the drawings anddetailed description herein are to be regarded in an illustrative ratherthan a restrictive manner, and are not intended to be limiting to theparticular forms and examples disclosed. On the contrary, included areany further modifications, changes, rearrangements, substitutions,alternatives, design choices, and embodiments apparent to those ofordinary skill in the art, without departing from the spirit and scopehereof, as defined by the following claims. Thus, it is intended thatthe following claims be interpreted to embrace all such furthermodifications, changes, rearrangements, substitutions, alternatives,design choices, and embodiments.

1. A DC to DC converter circuit, comprising: circuitry for generating aPWM waveform signal of the DC to DC converter responsive to an inputvoltage and a monitored output voltage, the circuitry further comprisinga high side switching transistor connected between the input voltage andthe phase node and a low side switching transistor connected between thephase node and ground; an output filter connected to the circuitry forgenerating the PWM waveform signal, the output filter including aninductor having a first side connected to the phase node and a secondside connected to an output voltage node; detection circuitry fordetecting zero current crossings in the inductor responsive to a voltageacross the high side switching transistor and a voltage across the lowside switching transistor; and wherein the detection circuitry detectsthe zero current crossing in the inductor using the turn on time of ahigh side switching transistor at a frequency range causing a turn ontime of a low side switching transistor to be too short to detect thezero current crossing in the inductor using the low side switchingtransistor.
 2. The DC to DC converter circuit of claim 1, wherein thedetection of the zero current crossings in the inductor enables the DCto DC converter to pass from a PWM mode to a PFM mode.
 3. The DC to DCconverter circuit of claim 1, wherein the detection circuitry furthercomprises: a first comparator for comparing a first voltage at the phasenode with a second voltage at the ground and generating a first outputresponsive thereto; a second comparator for comparing the first voltageat the phase node with a third voltage at an input voltage node andgenerating a second output responsive thereto; and a logical OR gateconnected to receive the first output and the second output and generatea third output responsive thereto, wherein the third output indicatesthe zero current crossings in the inductor.
 4. The DC to DC convertercircuit of claim 3, wherein the first comparator providing the firstoutput at a first logical level is responsive to a difference betweenthe first voltage and the second voltage going from a negative value toa positive value and further wherein the second comparator providing thesecond output at the first logical level is responsive to a differencebetween the first voltage and the third voltage going from a negativevalue to a positive value.
 5. The DC to DC converter circuit of claim 3,wherein the detection circuitry can detect the zero current crossings inthe inductor when the operating frequency of the DC to DC converteroperates at a frequency range causing a turn on time of the low sideswitching transistor to be too short for the first comparator to detectthe zero current crossings in the inductor.
 6. The DC to DC convertercircuit of claim 2, wherein the detection circuitry detects the zerocurrent crossings in the inductor using the turn on time of high sideswitching transistor at a frequency range causing a turn on time of thelow side switching transistor to be too short for the first comparatorto detect the zero current crossings in the inductor.
 7. Detectioncircuitry for detecting zero current crossings through an inductor of aDC to DC converter circuit, comprising: a first comparator for detectinga turn on time of a low side switching transistor by comparing a firstvoltage at the phase node with a second voltage at a ground node andgenerating a first output responsive thereto; a second comparator fordetecting a turn on time of a high side switching transistor bycomparing the first voltage at the phase node with a third voltage at aninput voltage and generating a second output responsive thereto; alogical OR gate connected to receive the first output and the secondoutput and generate a third output responsive thereto, wherein the thirdoutput indicates the zero current crossings in the inductor; and whereinthe detection circuitry can detect the zero current crossings in theinductor when the operating frequency of the DC to DC converter operatesat a frequency range causing a turn on time of the low side switchingtransistor to be too short for the first comparator to detect the zerocurrent crossings in the inductor.
 8. The detection circuit of claim 7,wherein the detection of the zero current crossings in the inductorenables the DC to DC converter to pass from a PWM mode to a PFM mode. 9.The detection circuit of claim 7, wherein the first comparator providingthe first output at a first logical level is responsive to a differencebetween the first voltage and the second voltage going from a negativevalue to a positive value and further wherein the second comparatorproviding the second output at the first logical level is responsive toa difference between the first voltage and the third voltage going froma negative value to a positive value.
 10. The detection circuit of claim7, wherein the detection circuitry detects the zero current crossings inthe inductor using the turn on time of high side switching transistor ata frequency range causing a turn on time of the low side switchingtransistor to be too short for the first comparator to detect the zerocurrent crossings in the inductor.
 11. A method for detecting zerocurrent crossings within a DC to DC converter circuit, comprising thesteps of: generating a PWM waveform signal of the DC to DC converterresponsive to an input voltage and a monitored output voltage; filteringthe PWM waveform signal through an output filter including and inductorto generate an output voltage; detecting zero current crossings in theinductor responsive to a voltage across both a high side switchingtransistor and a low side switching transistor when the operatingfrequency of the DC to DC converter operates at a frequency rangecausing a turn on time of the low side switching transistor to be tooshort to detect the zero current crossing in the inductor using only thelow side switching transistor.
 12. The method of claim 11, wherein thestep of detecting further comprises the steps of: detecting an “on”state of the low side switching transistor; detecting an “on” state ofthe high side switching transistor; and generating an indication of thezero current crossing responsive to the detection of the “on” state ofthe low side switching transistor and the “on” state of at least one ofthe high side switching transistor.
 13. The method of claim 11 furtherincluding the step of switching the DC to DC converter from a PWM modeto a PFM mode responsive to the detection of the zero current crossingsin the inductor.
 14. The method of claim 11, wherein the step ofdetecting further comprises the steps of: comparing a first voltage at aphase node on a first side of the low side switching transistor with asecond voltage at a ground node on a second side of the low sideswitching transistor; generating a first output responsive to thecomparison of the first voltage and the second voltage; comparing thefirst voltage at the phase node on a first side of the high sideswitching transistor with a third voltage at an input voltage node on asecond side of the high side switching transistor; and generating asecond output responsive to the comparison of the first voltage and thethird voltage; and logically ORing the first output and the secondoutput to generate a third output, wherein the third output indicatesthe zero current crossings in the inductor.
 15. The method of claim 14,wherein the step of generating the first output further comprises thestep of generating the first output at a first logical level responsiveto a difference between the first voltage and the second voltage goingfrom a negative value to a positive value.
 16. The method of claim 15,wherein the step of generating the second output further comprises thestep of generating the second output at the first logical levelresponsive to a difference between the first voltage and the thirdvoltage going from a negative value to a positive value.
 17. Detectioncircuitry for detecting zero current crossings through an inductor of aDC to DC converter circuit, comprising: a first comparator for detectinga turn on time of a low side switching transistor by comparing a firstvoltage at the phase node with a second voltage at a ground node andgenerating a first output responsive thereto; a second comparator fordetecting a turn on time of a high side switching transistor bycomparing the first voltage at the phase node with a third voltage at aninput voltage and generating a second output responsive thereto; alogical OR gate connected to receive the first output and the secondoutput and generate a third output responsive thereto, wherein the thirdoutput indicates the zero current crossings in the inductor; and whereinthe detection circuitry detects the zero current crossings in theinductor using the turn on time of high side switching transistor at afrequency range causing a turn on time of the low side switchingtransistor to be too short for the first comparator to detect the zerocurrent crossings in the inductor.